sidekick99 :
Is it worth it buying the retail version of windows 10??
I mean the oem version costs ~50$ and the retail version ~100$
If you plan on moving the OS to different hardware constantly then yes get the retail, if you plan on putting it on a computer and moving it to a new computer in 3-5 years then the oem system builder is just fine.
Now as asked where are you getting these prices from, this is too cheap to be legit.
The non-legit sources are selling you a pirated key (often from a large company microsoft licensing subscription). So IF the key even activates by microsoft, in a week, a month, 6 months when the rightful owner registers the key for their system then yours will no longer be activated.
Buy a legit key. If you get emailed a prodcut key then it is not legit!